Gas turbine engines with heat exchange recuperators are well known in the prior art. A recuperator is a heat exchanger which utilizes hot exhaust gases from the engine to heat the compressed air input from a compressor prior to insertion of the compressed air to the combustion chamber. Preheating the compressed air improves fuel efficiency of the engine in a number of ways.
The amount of heat which must be provided by the burners is reduced and therefore, the size of burner and fuel consumption is accordingly reduced. In addition, the pressure output from the compressor can be reduced since the addition of heat to the cold compressed air, increases the potential energy of the compressed air which is proportional to the product of pressure and temperature. Therefore, increasing pressure or increasing temperature have similar effect on the end result.
In general, the heat from exhaust gases is considered as a waste, however, in the case of military aircraft, the production of heat is a prime concern since the exhaust gas heat aids in identification and targeting of military aircraft using infrared tracking technology. Therefore, by reducing the heat of exhaust gases, the infrared signature of the aircraft can be reduced.
Prior art recuperators, however, suffer from significant advantages. In general, the prior art devices are large and heavy structures which render them completely unsuitable for aircraft use. Aircraft engines must maintain a minimal frontal area to minimize drag. The inclusion of large heat exchange ducts and recuperators increase the frontal area of aircraft engines to the extent that the gains acquired from use of a recuperator are lost in drag caused by the size of the engine.
As well, prior art recuperators include complex convoluted heat exchange ducting which increases internal resistance to air flow and adds operational load on the compressor. For example, including several heat exchange cross-flow pipes in an exhaust manifold will increase air flow resistance of the exhaust and air flow resistance into the combustion chamber simultaneously. The cumulative effect of a complex heat exchange recuperator will be to increase the load on the compressor and reduce the engine output thrust. As a result therefore, on balance the gains made through use of a conventional heat exchange recuperator are not significantly in excess of the losses in engine efficiency when all disadvantageous factors are included.
For example, U.S. Pat. No. 4,506,502 to Shapiro includes a large drum honeycomb heat exchanger which is heavy and expensive to manufacture, in addition to increasing air flow resistance within the engine. The combination of these factors renders it completely unsuitable for aircraft use.
In a like manner, U.S. Pat. No. 5,119,624 to McKenna includes large heat exchangers inserted in exhaust ports of an engine to preheat the compressed air. Such ducts increase the size and weight of the engine rendering it unsuitable for aircraft use. However, where gas turbine engines are used as auxiliary power units for ground level electrical generation for example, such arrangements may be useful. A further example of large size heat exchangers on gas turbine engines are shown in U.S. Pat. No. 4,141,212 to Koschier and U.S. Pat. No. 4,974,413 to Szego.
An improvement over such large size ducting is shown in U.S. Pat. No. 5,253,472 to Dev wherein the heat exchange recuperator is provided generally within the conventional outward dimensions of a gas turbine engine. A significant disadvantage of the Dev system is the large number of small heat transfer tubes through which the compressed air must be conducted and the number of turns that the compressed air is forced to go through before entering the combustion chamber. The large number of small heat transferred tubes significantly increases air flow resistance. The complicated convoluted path followed by the compressed air through multiple bends while beneficially increasing heat transfer, also significantly increases internal air flow resistance. Both of these factors significantly increase the air flow resistance and operational load on the compressor and engine as a whole increasing fuel consumption. It is expected that the increase in fuel consumption due to internal air flow resistance practically overrides any decrease in fuel consumption gained from preheating the compressed air.
It is an object of the invention therefore, to obtain the fuel consumption reductions available through use of a heat exchange recuperator while impeding air flow resistance within the engine as little as possible.
It is a further object of the invention to provide a heat exchange recuperator which does not increase the outward size and dimension of the gas turbine engineer.
It is a further object of the invention to provide a heat exchange recuperator which adds minimal weight increase and engine complexity to the gas turbine engine.